Energy systems used to power unmanned undersea vehicles (UUVs) have unique requirements. Undersea vehicles are closed systems; all power generation equipment and fuel must be stored within the vehicle for the duration of the underwater voyage. Thus, minimizing the weight, volume, and parasitic power requirements of the fuels and associated hardware is essential, especially in small vehicles. Also, unmanned undersea vehicles generally travel at low speeds, so they cannot generate substantial hydrodynamic lift. Therefore, these vehicles need to maintain neutral buoyancy, proper center of buoyancy/center of gravity separation, and longitudinal and transverse trim. However, maintaining vehicle balance is difficult with open cycle systems which pump in seawater for purposes such as providing reactant and reaction product waste disposal, in which the volume of water in and out varies. Additionally, an energy system in an undersea vehicle must be operable at various orientations or angles of pitch and roll.
Unmanned undersea vehicles are typically powered by batteries. However, fuel cell power systems have been developed to the point where they may soon become a viable alternative power source to batteries. Fuel cells convert the chemical energy of a fuel and oxidizer into electricity by electrochemical reactions. For example, in some types of fuel cells, externally supplied hydrogen, H.sub.2, and oxygen, O.sub.2, are converted into water and energy. Proton exchange membrane (PEM) fuel cells, which use H.sub.2 and O.sub.2, are emerging as a likely replacement for batteries as an efficient power source for undersea vehicles. PEM fuel cells have a high efficiency at low temperatures and pressures, have no liquid electrolyte, and can operate at high power densities. Also, PEM fuel cell technology is sufficiently well developed to provide a viable power source for UUVs.
However, all fuel cells require external reactant supplies. That is, all the fuel and oxidizer, such as hydrogen and oxygen, to be used by a fuel cell during a voyage of an undersea vehicle must be stored and/or generated in the vehicle. Thus, to maximize the specific energy and energy density, it is necessary to minimize the volume and mass of the reactants to be stored.
Hydrogen can be stored as a liquid in a cryogenic tank. However, cryogenic tank storage of liquid hydrogen (LH2) has disadvantages. LH2 has a low weight, but it also has a low specific gravity, which results in a disproportionately large volume. Also, LH2 is dangerous. The risk of boiloff is significant, particularly in an undersea vehicle. Thus, alternate methods of storing or generating hydrogen are needed before fuel cells can become a viable power source for undersea vehicles.
Hydrogen can be generated by the reaction of various hydrides with water, known as hydrolysis. A typical reaction is the following: EQU M.sub.x H.sub.y +zH.sub.2 O.fwdarw.M.sub.x (OH).sub.z +(z/2+y/2)H.sub.2.
M denotes one or more metals bonded to the hydrogen to form the hydride. The reaction is exothermic. Table 1 lists a variety of hydrides and the hydrolysis reaction associated with each hydride.
TABLE 1 ______________________________________ Hydride Reaction ______________________________________ Diborane B.sub.2 H.sub.6 + 6H.sub.2 O .fwdarw. 2H.sub.3 BO.sub.3 + 6H.sub.2 Lithium Borohydride LiBH.sub.4 + 2H.sub.2 O .fwdarw. LiBO.sub.2 + 4H.sub.2 Lithium Hydride LiH + H.sub.2 O .fwdarw. LiOH + H.sub.2 Trisilane Si.sub.3 H.sub.8 + 6H.sub.2 O .fwdarw. 3SiO.sub.2 + 10H.sub.2 Sodium Borohydride NaBH.sub.4 + 2H.sub.2 O .fwdarw. NaBO.sub.2 + 4H.sub.2 Lithium Aluminum Hydride LiAlH.sub.4 + 5/2H.sub.2 O .fwdarw. LiOH + 1/2Al.sub.2 O.sub.3 + 4H.sub.2 Magnesium Hydride MgH.sub.2 + 2H.sub.2 O .fwdarw. Mg(OH).sub.2 2H.sub.2 Calcium Hydride CaH.sub.2 + 2H.sub.2 O .fwdarw. Ca(OH).sub.2 + 2H.sub.2 Sodium Hydride NaH + H.sub.2 O .fwdarw. NaOH + H.sub.2 ______________________________________
Hydrids are routinely hydrolyzed to generate hydrogen for applications ranging from filling weather balloons to providing laboratory gas supplies. In conventional methods, a measured amount of hydride is introduced into water. The water is provided in excess of the stoichiometric amount of water needed for the hydrolysis reaction to occur. This excess of water renders such methods unsuitable for use in UUVs, where there is no room for the storage of this excess water.
In another known hydrogen generator, the Kipp Generator, a column of hydride is inverted in water, such that some water enters the bottom of the column and reacts with the hydride there. Hydrogen generated in the reaction rises to the top of the column where it is removed through a control valve. The water level in the column, and hence the reaction rate, is controlled by controlling the pressure exerted by the hydrogen over the column. The column must remain in an upright orientation, however, for the reaction to remain controllable. Tilting of the column, such as would occur in undersea vehicles, would tilt the water level relative to the hydrides in the column, causing unwanted reactions as the water contacted the unreacted hydrides.
In an additional known method for generating hydrogen, a humidified gas is blown through a bed of hydride to distribute water vapor. However, the power required to blow gas through the bed of hydride may, in some instances, be excessive, and the noise of the blower may, in some applications, be undesirable. In a further known method, a reaction vessel is provided having a wick disposed therein to transport water to unreacted hydride. However, control of the reaction rate is difficult with this method. Thus, these systems of generating hydrogen are not suitable for use in undersea vehicles.